The present invention relates to an image forming apparatus such as a printer, a facsimile and a copying machine for forming an image using electrophotographic technology. Particularly, the present invention relates to cleaning technique of a transfer member for transferring an image on a recording medium such as paper.
Generally, an image forming apparatus using electrophotographic technology is provided with a photoconductor drum provided with a photosensitive layer on the peripheral surface as a latent image carrier, charging means for uniformly charging the peripheral surface of the photoconductor drum, exposure means for forming an electrostatic latent image by selectively exposing the peripheral surface uniformly charged by the charging means, development means for applying charged toner as developer to an electrostatic latent image formed by the exposure means to form a visible image (a toner image) and a transfer device for transferring a toner image developed by the development means on a recording medium such as paper.
For example, a conventional type image forming apparatus disclosed in Unexamined Japanese Patent Publication No. Hei 1-319078 shown in FIG. 5 is known.
The image forming apparatus is provided with an charging device 2 for uniformly charging a photosensitive layer on the surface of a cylindrical image carrier, an exposing section 3 for irradiating optical information such as a laser beam on the charged surface to form an electrostatic latent image, a developing machine 4 for supplying toner to the latent image to visualize the image, a transfer roller 5 abutted against the above image carrier 1 as a transfer member for transferring a toner image on a recording medium fed to the nipping part through a supplying passage 6, a power source 7 for applying developing bias to a developing part of the developing machine 4 and a power source 8 for applying transfer bias to the transfer roller 5 around the cylindrical image carrier 1 extended in a direction perpendicular to the surface of paper and rotated in a direction shown by an arrow A.
An image forming process by the image forming apparatus is as follows:
First, the photosensitive layer of the image carrier 1 is charged by a corona charger 2 so that the photosensitive layer is -700 V.
Next, the voltage of an irradiated area is attenuated to -100 V by irradiating optical information by the exposing section 3 and an electrostatic latent image is formed in the part.
Developing bias of -500 V is applied to the developing machine 4 by the power source 7, toner charged in negative polarity adheres to the part in which electric potential is attenuated by the reversal development and thereby a toner image is formed.
In the meantime, transfer material not shown supplied through the supplying passage 6 reaches the nipping portion between the image carrier 1 and the transfer roller 5 at the same timing as the above toner image, transfer bias of +500 V is applied to the transfer roller 5 by the power source 8 when the transfer material passes and a toner image is transferred on the transfer material.
The conventional type image forming apparatus is constituted so that when optical information is not irradiated on the image carrier 1, the charging device 2 is stopped so that the surface potential of the image carrier 1 is 0 V, when an uncharged area in which surface potential is zero is located in the nipping portion between the image carrier 1 and the transfer roller 5, that is, when no transfer material passes, bias of the same polarity as toner and of polarity reverse to the above transfer bias, namely -500 V, is applied to the transfer roller 5, hereby, toner which adheres to the transfer roller 5 is moved to the image carrier 1 and the transfer roller 5 is cleaned up.
There is a problem that as the above conventional type transfer device is constituted so that positive voltage of +500 V is applied to the transfer roller 5 when a toner image is transferred on transfer material and negative voltage of -500 V is applied to the transfer roller when the transfer roller 5 is cleaned up, a high voltage relay and others for switching applied voltage are required and the structure of a high voltage power supply unit is complicated.